feat: add conflict-fix leases and stale-head protection (Closes #399)

Introduce structured PR work leases so author conflict-fix pushes cannot
race reviewer validation, approval, or merge on a moving head SHA.

- pr_work_lease.py: parse/acquire leases, push and reviewer mutation gates
- gitea_acquire_conflict_fix_lease, gitea_assess_conflict_fix_push MCP tools
- Enforce reviewed head SHA on mark_final_review_decision, submit_pr_review, merge_pr
- Final-report rules and workflow sections 20A / 26B
- tests/test_pr_work_lease.py (14 cases)

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -732,6 +732,24 @@ The final report must identify:
 * whether same-PR merge continuation was allowed
 * whether the run stopped as required
 
+## 26B. Conflict-fix lease and stale-head protection (#399)
+
+Before validating, approving, or merging a PR:
+
+1. Check for an active conflict-fix lease on the PR; stop if one is active.
+2. Pin `expected_head_sha` before validation and pass it to
+   `gitea_mark_final_review_decision`, `gitea_submit_pr_review`, and
+   `gitea_merge_pr`.
+3. Re-fetch live PR head immediately before approval and merge; refuse when
+   live head differs from the reviewed SHA.
+
+Final reports must state:
+
+* reviewed head SHA
+* final live head SHA before approval
+* final live head SHA before merge
+* whether any push occurred during validation
+
 ## 27. Merge rules
 
 Before merge, rerun fresh live checks: